Certain signal processing systems simultaneously detect a plurality of analog information signals. Examples of these types of systems include computerized axial tomography (hereinafter “CT”) scanners that are used to image the internal features of an object (e.g., a human body) by exposing the object to a preselected amount and type of radiation.
For example, certain CT scanners (called third generation scanners) typically include an X-ray source and an X-ray detector system secured respectively on diametrically-opposed sides of an annular-shaped disk, which is rotatable within a gantry support. The X-ray detector system includes an array of detectors. During a scanning procedure, the object being scanned is positioned within an opening of the annular-shaped disk (which continuously rotates about an axis of rotation), and X-rays pass from the X-ray source through the object and to the X-ray detector system.
In one type of system, the signals generated by the X-ray detector system are typically provided to the summing node input of an array of delta-sigma converters, such as the delta-sigma converters described in U.S. Pat. No. 6,252,531, entitled “Data acquisition system using delta-sigma analog-to-digital signal converters”.
One type of X-ray detector system includes an array of detectors positioned as a single row or multiple rows in the shape of an arc of a circle or surface of a sphere having a center of curvature at the point (i.e., the “focal spot”) where the radiation emanates from the X-ray source. The X-ray source and array of X-ray detectors are all positioned so that the X-ray paths between the source and each detector all lie in the same plane (i.e., the “slice plane” or “scanning plane”) or planes which are substantially normal to the rotation axis of the annular-shaped disk.
The X-rays that are detected by a single X-ray detector at a measuring instant during a scan is considered a “ray.” Because the ray paths originate from substantially a point source and extend at different angles to the detectors, the ray paths resemble a fan or cone, and thus the terms “fan” beam or “cone” beam are frequently used to describe all of the ray paths at any one instant of time. As a ray passes through the object being scanned, it is partially attenuated, thus generating a single intensity measurement as a function of the attenuation (i.e., the density of the mass in that path).
In other types of CT scanners (referred to as fourth generation scanners), the detection system comprises a circular array of detectors secured on and at equiangular positions around the gantry support, equidistant from the center of rotation of the disk, such that the source rotates relative to the detectors. For these scanners, the fan beam is defined as the ray paths from the rotating source to each detector where the point of convergence of each fan beam is the corresponding detector.
There are various types of detectors used in CT scanners, including solid state detectors (e.g., cadmium tungstate detectors, each of which includes a scintillation crystal or layer of ceramic material and a photodiode) and gas-type detectors (e.g., Xenon detectors). The X-ray source may provide a continuous wave or a pulsed X-ray beam.
Unfortunately, these detectors typically have a large detector diode capacitance (i.e., 50–200 picofarads). Since it is desirable to reduce the power consumption in multi-channel CT scanners (which typically include 50,000–60,000 channels), this high capacitance proves problematic when coupling detectors to the summing node input of a delta-sigma converter. Specifically, this high detector capacitance results in a high gain-bandwidth product and, therefore, an unacceptably high (i.e.,>one milliwatt) per-channel power consumption.